Remotely positionable Magnetic Resonance Imaging (MRI) devices have many advantages which stem primarily from the fact they can produce images of in situ biological tissue without requiring that the device surround and confine the tissue being imaged. By virtue of being remotely positionable, however, they also have unique characteristics which must be taken into consideration. For instance, a remotely positionable MRI device generates an inhomogeneous magnetic field. This fact, alone, has a very significant effect on how the device is designed and how it operates.
In order to obtain useable images of in situ tissue with a remotely positioned MRI device, there needs to be a defined measurement region within the inhomogeneous magnetic field which is specifically configured for NMR imaging. More specifically, it is the measurement region which coincidently covers the tissue that is to be imaged during the operation of the MRI device. Although the specific configuration for the measurement region can be varied somewhat according to the desires of the operator, for medical applications the measurement region will typically have a nearly flat planar configuration. Further, the measurement region will typically be relatively thin, with an overall thickness that is about half a centimeter (0.5 cm). This particular configuration is preferred so that a realistic two dimensional picture of in situ tissue in the plane of the measurement region can be created.
As is well known to skilled artisans, Magnetic Resonance Imaging (MRI) is a diagnostic procedure that is extensively used in the medical field to noninvasively image internal biological tissue. As is also well known, MRI relies on the nuclear magnetic resonance (NMR) of nuclei, and the fact that when tissue nuclei are placed in the environment of a strong external magnetic field they will each assume a discrete energy state. Further, NMR takes advantage of the fact that while tissue nuclei are influenced by an external magnetic field, radio frequency (RF) energy will induce changes in their energy states to generate spin echo signals which are characteristic of the tissue. The RF radiation which is most effective for inducing such changes has a particular frequency (known as the Larmor frequency) which depends on the magnitude of the magnetic field at the location of the nucleus.
In one important respect, the present invention is quite different from conventional MRI systems. As implied above, this difference arises from the fact that NMR is normally performed in nearly homogeneous magnetic fields where there is a nearly constant field strength. Thus, with essentially no field strength gradient the range or bandwidth of Larmor frequencies necessary for NMR is very small. The system and method of the present invention, however, uses an inhomogeneous magnetic field. Inhomogeneous magnetic fields, unlike homogeneous magnetic fields, have a field strength gradient (G.sub.z) that dominates the behavior of nuclei in the magnetic field. Consequently, because there is a continual change in the field strength, due to G.sub.z, inhomogeneous magnetic fields require a much greater bandwidth of Larmor frequencies to tilt and refocus nuclei in a slice of in situ biological tissue during MRI. For an effective MRI procedure using an inhomogeneous magnetic field, this is a factor which must be dealt with.
The requirement for an increased bandwidth of RF radiation arises because, in the presence of a field strength gradient each nucleus along the direction of the gradient will be influenced by a slightly different field strength. Thus, each nucleus will have a slightly different Larmor frequency. The consequence of this is that, as more nuclei are to be influenced by different field strengths during an MRI procedure, the bandwidth of the RF radiation must necessarily be broadened to include all of the required Larmor frequencies. For a measurement region having a thickness of approximately 0.5 cm, and a field strength gradient of approximately two hundred Gauss per centimeter (200 Gauss/cm), the bandwidth will be about four hundred and twenty KiloHertz (420 KHz). A broadened bandwidth introduces additional noise which lowers the signal-to-noise ratio (SNR) and makes detection of the spin echoes more difficult. Also, large bulky equipment is needed to generate high bandwidth pulses of RF energy. Thus narrower bandwidths are preferable. For a complete disclosure and discussion of the phenomenon which impact NMR using inhomogeneous magnetic fields, refer to U.S. Pat. No. 5,304,930 to issue on Apr. 19, 1994, for an invention entitled "Remotely Positioned MRI System" which is assigned to the same assignee as the present invention, and which is incorporated herein by reference.
An additional consideration which is pertinent to MRI procedures wherein inhomogeneous magnetic fields are used concerns the time period during which the NMR data is recorded. Specifically, for NMR there are two time constants which are characteristic of relaxation mechanisms. The first time constant of interest is the time required for the nuclear magnetic moments to return to equilibrium after having been tilted by an excitation and preparation pulse sequence, such as an initial 90.degree. pulse. This time constant (T.sub.1), also commonly referred to as the spin-lattice relaxation time, is the time for longitudinal magnetization of the nuclei to be restored. The other time constant of interest is the time over which transverse components of the net magnetic moments randomly dephase in phase coherence after they have been tilted. This second time constant (T.sub.2) is commonly referred to as the spin-spin relaxation time. T.sub.1 is not equal to T.sub.2. In fact, in biological tissue, T.sub.1 is often approximately one order of magnitude (10.times.) greater than T.sub.2. A related relaxation time of importance in MRI instruments using inhomogeneous fields is T.sub.2 star (T.sub.2 *). T.sub.2 * is the time constant that characterizes the rate at which transverse components loose their phase coherence due to the range of Larmor frequencies. As is known in the art, the dephasing characterized by T.sub.2 * may be reversed by applying a refocussing pulse to generate a spin echo. By adjusting the excitation and data recording timing sequences in an MRI instrument, it is well known that the resultant image can be characterized as either T.sub.1 weighted or T.sub.2 weighted.
It is known that once nuclei have been tilted by a 90.degree. pulse at their Larmor frequency they must wait a period of time on the order of T.sub.1 before the longitudinal magnetization is substantially restored. Consequently, since T.sub.1 is approximately ten times longer that T.sub.2, if spin echo signals are refocussed and recorded during a time period roughly equal to T.sub.2, each refocussing and recording epoch should be followed by a waiting time period which is approximately equal to 9T.sub.2. During this hiatus, no further tilting or recording of refocussed spin echos is accomplished in order to allow restoration of the longitudinal magnetic moment. In the prior art, several techniques are disclosed to make use of this waiting time. In an article published in the Journal of Magnetic Resonance, Vol. 33 (1979) pp. 83-106 and entitled "Sensitivity and Performance Time in NMR Imaging", Brunner and R. R. Ernst demonstrate that refocussing and data recording of separate images that are adjacent and parallel to the original image can proceed during the aforementioned waiting time of the original image. Also, a method and apparatus for achieving a similar impulse like excitation and data acquisition sequence is disclosed by Ian R. Yoy in U.S. Pat. No. 4,558,278. Additionally, the role of data recordation times in efficiently computing T.sub.1 and T.sub.2 relaxations for refocussed spin echo pulse trains is disclosed in U.S. Pat. No. 5,055,788 which issued on Oct. 8, 1991 to Kleinberg et al. for an invention entitled Borehole Measurement of NMR Characteristics of Earth Formations.
As stated above, however, in an inhomogeneous field MRI system, the imaging of even a single image slice may require a large RF bandwidth. Stated differently, in light of the inhomogeneous field contemplated for the present invention, the total tissue slice for an inhomogeneous field device necessarily consists of a single image slice having a thickness on the order of 0.5 cm. The present invention recognizes that by imaging thin subslices of the total tissue slice being imaged, and by using different Larmor frequencies for each thin subslice, several advantages are obtained. First, the bandwidth of the RF pulse required for imaging each thin subslice is significantly reduced from the bandwidth required to image the total tissue slice. Second, after recording spin echoes for the image of one subslice, another subslice can be immediately imaged without waiting for T.sub.1 of the previously imaged subslice to expire. Third, the recorded subslice images can be combined to construct a single image of the total tissue slice that is comparable in quality to the image which would result if the total tissue slice were simultaneously imaged using the larger bandwidth for the RF pulse.
In light of the above, it is an object of the present invention to provide a method and device for imaging in situ biological tissue which combines the images from several thin subslices to construct a single MRI image of the total slice of tissue. Another object of the present invention is to provide a method and device for imaging in situ biological tissue which can vary the number of subslices, and the time for recording spin echoes from each subslice, in order to T.sub.1 or T.sub.2 weight the resultant image of the total slice of tissue. Yet another object of the present invention is to provide for a method and device for imaging in situ biological tissue which effectively records spin echoes from a subslice of tissue during T.sub.1 relaxation of an earlier recorded subslice. Still another object of the present invention is to provide a method for imaging in situ biological tissue when using a remotely positioned MRI device, which is easy to use, simple to implement, and comparatively cost effective.